The invention relates to a single-phase synchronous motor comprising a two-pole permanent-magnet rotor between two electromagnetically excited stator coils.
A single-phase synchronous motor with a permanent-magnet rotor is known from the magazine ETZ, Vol. 30, 1978, No. 2, pages 56 to 60 and DE-OS No. 32 15 376. This cylindrical permanent-magnet rotor is magnetised in diametrally parallel directions. The magnetizing coils are arranged on the U-shaped stator iron. Between the stator poles a two-pole stator field is formed. The inner field lines in the rotor of this motor are curved in such a way that poles with a high field density are formed, the two opposite pole faces each being limited to an angle of the order of magnitude of 65.degree. on the rotor periphery. This results in concentrated magnet poles which urge the rotor towards the smallest air gap between the rotor and the asymmetric stator.
For reasons of economy it may be favourable to arrange mutually spaced permanent-magnet bodies on a symmetrical iron core (article by Mr. Karl Ruschmeyer, "Motoren und Generatoren mit Dauermagneten", Vol. 123, Kontakte und Studium, edited by Prof. Dr. Ing. Wilfried J. Bartz, Expert-Verlag, 1983, pages 36 and 37). The known single-phase synchronous motors described in ETZ, Vol. 30 must comply with a specific requirement in order to obtain self-starting. This requirement is that when the motor is switched off the rotor is rotated in a specific position relative to the stator poles. This is achieved by means of the "detent torque". In addition, an angle of asymmetry must be defined, whichis an angle between the orientation of the rotor field and the stator field when the motor is stationary and which is dictated by the motor construction.
From DE-AS No. 12 61 235 it is known to construct the rotor of a low-power synchronous motor from two permanent-magnetic elements between which a thin sheet is interposed. This rotor has cylindrical peripheral portions and is flattened at the location of the thin sheet. A single-phase synchronous motor with such a cylindrical construction has a poor starting behaviour because the angle of a symmetry which can be obtained with this cylindrical construction is comparatively small.